Questions and answers about the Ex p pressurized enclosure degree of protection

The pressurized enclosure (Ex p) type of protection is robust, durable, user-friendly, space- and weight-saving, and cost-effective. However, many users are unsure about how to handle the pressurized enclosure (Ex p) type of protection. Are you one of them? Then you will find answers to frequently asked questions about this type of protection.

What is the Ex p pressurized enclosure degree of protection?

The Ex p system can be used in gas hazardous areas in Zones 1 and 2, as well as in dust hazardous areas in Zones 21 and 22. The control and monitoring systems in the pressurized enclosure type of protection always consist of an Ex p control unit, Ex p pressure monitors and Ex p purge valves with digital and proportional control.

With this principle, a closed enclosure or a larger cabinet, which is inside the hazardous area, is purged with ignition protection gas until the combustible gas mixture inside is diluted enough that an Ex-free area is created. After this "pre-purging phase", a slight overpressure with respect to the surrounding Ex atmosphere is generated and monitored. Following this, non-explosion-protected equipment can be safely connected inside the enclosure or cabinet.

Based on time intervals and pressure analyses, the purge valves automatically control and transport ignition protection gases, such as compressed air, into the inside of the Ex p enclosure. Additionally, a temperature monitoring and control system is integrated inside the enclosure to provide adequate cooling. In doing so, the systems cover requirements for extremely small (10 litre volume) to extremely large Ex p cabinets with volumes of 4000 litres or more.

How does the degree of protection work?

Typical Ex p pressurized enclosures are operated in three separate steps:

Preparation phase: Defined quantities of compressed air or inert gases are transported into the Ex p enclosure via the purge valve and an overpressure is generated. Once this is achieved and the Ex p control unit detects this, the "pre-purging phase” begins; this is regulated via the Ex p control unit.

Pre-purging phase: Before starting up the equipment located inside the cabinet, any explosive mixture that may be present must be removed from the enclosure. The purge valve opens once the preset internal enclosure pressure has been reached. Next, thorough purging is performed using ignition protection gas that flows through the additional opening in the air outlet valve. These air outlet valves (pressure monitors) are positioned in different locations in order to guarantee controlled flow behaviour. The extremely short purge times at very low pressure loads (0.002 bar) are particularly advantageous. This enables the wall thickness of the cabinet walls to be reduced to less than 2 mm. This, in turn, lowers costs. Once the pre-purging phase is complete, the purge valve closes and the purge flow stops. The internal enclosure pressure returns to the preset value and the air outlet valve closes.

Operating phase: Normal operation begins and the control unit monitors and regulates the internal enclosure pressure in order to prevent explosive gases or dusts from entering the enclosure. The purge valve automatically compensates for any leakage losses.

What advantages does the Ex p degree of protection offer?

The Ex p degree of protection is extremely robust, lasts a long time (service lives spanning 15 years are not uncommon) and is very user-friendly. This means that commercially available Ex e enclosures, as well as smaller and larger standard enclosures, can be used. Furthermore, the enclosures are significantly lighter than other degrees of protection. For instance, a typical Ex d enclosure weighs 180 kilograms; in contrast, a corresponding Ex p enclosure weighs just 40 kilograms. The bushings leading from inside the enclosure to the outside also do not need to feature the Ex d degree of protection. Ex e cable glands can be used, even for pipes and hoses. Use is now possible within a broad temperature range (from -30 to +60 °C) because the units can be air conditioned.

Which enclosures can I use?

The simplest way to get started with the Ex p degree of protection is to use prefabricated units. Your own devices can also be used in these. It is also possible to provide a U-certified Ex p empty enclosure. When doing so, we recommend selecting an enclosure with standard dimensions. In principle, another option is also to produce or commission an empty enclosure, purchase a suitable Ex p system and combine the components yourself.

In this case, when obtaining overall approval from a test body, qualified expert or approved company, the "semi-finished overall product" must be subjected to all required inspections. During this process, it is helpful if the empty enclosure used with the Ex p system is U-certified equipment. If the empty enclosure is not U-certified equipment, the entire surface must also be checked in accordance with the requirements of Ex p standard IEC 60079-2.

Is re-approval required if modifications are made to the system, for example an additional attachment screw connection?

That depends. Re-approval is not required for screw connections or two additional terminals. However, the Ex p enclosure must be re-assessed if there are new developments, such as if operating terminals are installed. If an old panel is simply being replaced with a new one, looking at the documentation of the original Ex p cabinet is sufficient in certain circumstances to see if there are limitations regarding replacement.

If additional devices are added to the interior, checks must also be performed in addition to the approval, for example, to see whether the purging phase is still correct given that the internal volume has been reduced.

It is therefore recommended first to clarify with the test body or manufacturer whether the built-in components affect the temperature or purging processes. Furthermore, there are no repair signs or similar for an Ex p cabinet. All modifications require re-approval with a type plate.

What about energy stores such as power supplies or frequency transformers in Zone 1/21? Are these excluded from being considered as installed devices?

When using energy stores, it must be ensured that the stored energy is dissipated in case of error before the doors of the Ex p cabinet are opened. Buffer batteries (typically 3 V) in Ex p cabinets are not considered to be dangerous; larger energy stores must each be assessed.

What is U-certified equipment?

The "U" stands for "unvollständig" (incomplete). This means that U-certified equipment refers to devices that require additional explosion protection. For example, a U-certified connection terminal still requires IP protection. The equipment installed at a later point must be assessed, even for a U-certified Ex p enclosure.

Are there limitations to the performances that can be achieved?

In principle, pressurized enclosures are not limited in the performances that can be achieved. It only depends on how large an Ex p cabinet or the Ex p applications are. It must be noted that Ex p applications are inside enclosures that are closed and therefore sealed. Power dissipation, i.e. heat flows, must be removed. There are various air conditioning systems available for this purpose. With regard to high voltages and currents, care must also be taken to ensure that the distances between the components comply with the relevant standards.

Does the Ex p enclosure always need to be built as new?

No. If the system requiring protection already has a protective enclosure which is suitable according to an initial examination, this can also be used. This is normal practice, for example with large Ex p motors, because the machine enclosure is already suitable and stable enough.

Does every Ex p application have to be fully re-certified by a test body?

This depends on whether the manufacturer, i.e. the person responsible for manufacturing or finally checking the complete Ex p application, has the authorisation required to certify it too. This is the case at R. STAHL. Therefore, the company can check the Ex p applications and provide valid type plates.

What happens if I only have an Ex p system and no Ex p certificate?

This happens frequently, generally with system engineers who manufacture fully functional control systems and use surrounding enclosures that are already suitable. Retrospectively, a test body can be called in or the manufacturer of the Ex p system can be asked whether they can perform the final inspections and procure a type plate for placing on the market.

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